Reimplement checkout item removal. All existing tests now pass

checkout.rb

@@ -13,10 +13,21 @@ class Checkout
 
     raise InvalidItem unless product
 
-    product_in_checkout = new_or_existing_product_in_checkout(product)
-    product_in_checkout.add_quantity(quantity)
+    new_or_existing_product_in_checkout(product).add_quantity(quantity)
+  end
 
-    @items.push product_in_checkout
+  def remove(product_id:, quantity: 1)
+    product = @shop.product_by_id(product_id)
+
+    raise InvalidItem unless product
+
+    product_in_checkout = existing_product_in_checkout(product)
+
+    return false unless product_in_checkout
+
+    product_in_checkout.remove_quantity(quantity)
+
+    trim_empty_items
   end
 
   def items
@@ -37,7 +48,21 @@ class Checkout
 
   private
 
+  def existing_product_in_checkout(product)
+    @items.find { |item| item.product_id == product.id }
+  end
+
   def new_or_existing_product_in_checkout(product)
-    @items.find { |item| item.product_id == product.id } || ProductInCheckout.new(product: product)
+    product_in_checkout = existing_product_in_checkout(product)
+    unless product_in_checkout
+      product_in_checkout = ProductInCheckout.new(product: product)
+      @items.push(product_in_checkout)
+    end
+
+    product_in_checkout
+  end
+
+  def trim_empty_items
+    @items = @items.filter { |item| item.quantity > 0 }
   end
 end